Cryogenic fluids such as oxygen, nitrogen, helium, hydrogen or argon are of great industrial importance, especially in the electronics field.
Thus, it is common practice to use nitrogen for inerting or cooling printed circuits during their manufacturing process and helium is often employed for cooling hot optical fibres.
At the present time, the various cryogenic fluids are obtained by cryogenic distillation from ambient air or from gas mixtures containing them or by non-cryogenic separation techniques, for example by pressure swing adsorption processes, usually called PSA processes, or by membrane permeation.
However, for some applications, especially in the electronics field, the cryogenic fluid must be of high purity, that is to say it must contain minimal amounts of impurities and other undesirable contaminants, in order to prevent these impurities from causing undesirable physico-chemical reactions or reactions incompatible with the intended aim.
Thus, it is usual to purify the cryogenic fluids very thoroughly, that is to say down to impurity levels of less than a few tens of ppb (parts per billion) or even to 1 ppb.
Up to the present time, many methods of purifying cryogenic fluids have already been proposed.
Thus, mention may be made of document EP A-662,595 which describes a process for preparing high-purity liquid nitrogen, in which process the carbon monoxide, oxygen and hydrogen impurities present in the liquid nitrogen are removed by adsorption on a zeolitic material or a material of the porous metal oxide type.
Furthermore, document U.S. Pat. No. 4,746,332 describes the removal of carbon monoxide present in liquid nitrogen by adsorption on a type 5A zeolite.
Moreover, EP-A-590,946 teaches prepurification, using a TSA (Temperature Swing Adsorption) type process, of carbon monoxide present in gaseous nitrogen at a temperature of 90 K. to 150 K., followed by ultrapurification by subsequent distillation of the nitrogen thus prepurified.
Document EP-A-750,933, for its part, relates to removal of carbon monoxide and oxygen which are present in liquid or gaseous nitrogen or argon by adsorption on a transition metal oxide or oxides, for example a hopcalite.
Moreover, document U.S. Pat. No. 4,717,406 teaches the purification of a stream by mechanical filtration and adsorption on an adsorbent, especially of the molecular-sieve, activated-charcoal or silica type.
Document WO-A-98/28226 recommends purifying a cryogenic fluid, such as helium, hydrogen or argon, of its impurities by mechanical filtration of the impurities in the form of crystals and adsorption of the impurities in dissolved or gaseous form, so as to obtain a purified fluid containing less than 1 ppb of impurities.
Furthermore document U.S. Pat. No. 4,425,143 describes a zeolite having enhanced adsorption performance and being characterized by a high Si/Al ratio in order to withstand acids, this zeolite being free of Fe.sub.2 O.sub.3.
Document EP-A-747,118 teaches the removal of oxygen impurities from an inert gas by means of a support impregnated with an alkali or alkaline-earth metal oxide.
Finally, document U.S. Pat. No. 3,597,169 relates to a process for removing methane impurities contained in liquid oxygen by employing a zeolite X highly exchanged with calcium or silver cations.
In practice, the adsorbent particles used in these various processes usually have an average size of about 2 mm to 5 mm.
However, although these processes allow certain impurities contained in the cryogenic fluids, especially such as carbon monoxide, carbon dioxide or oxygen impurities, to be removed relatively well, it appears that the problem of effective removal of certain other impurities likely to be present in relatively large amounts in cryogenic fluids have not been solved hitherto, or have only been solved incompletely.